This invention relates to fluidized bed combustion (FBC) systems, and more particularly to a bubbling fluidized bed (BFB) combustion system that is particularly well suited for the purpose of effecting the combustion therein of oil shale and generating steam thereby.
The use of fluidized beds is widespread in industry. They have been used for such diverse purposes as coal gasification, chemical pulping, gas phase polymerization and catalytic cracking. It has also been known in the prior art to provide fluidized beds of various types in the generation of steam. In this regard, one convenient method of differentiating between such types of fluidized bed combustion systems is by the nature of the fluidization that takes place therein. As employed in this context, the term "fluidization" refers to the manner in which solid particulate material is provided with a free-flowing, fluid like behavior. To this end, as a fluidizing gas is made to pass vertically in a fluidized bed combustion system through a bed of solid particles, such a flow of gases produces forces that tend to separate the solid particles from one another. At low gas velocities such forces can be insufficient to cause the solid particles to separate from one another such that the solid particles tend to remain in contact with one another, i.e., there is still a tendency for the particles to resist movement therebetween. When such a condition exists, it is referred to as a fixed bed and combustion systems employing this particular technique are referred to as fixed bed fluidized bed combustion systems. On the other hand, as the velocity of the fluidizing gas is increased, a point is reached wherein such velocity is sufficient to produce forces, acting upon the solid particles, adequate to cause separation of the solid particles. When this occurs, the bed of solid particles becomes fluidized in that a gas cushion between the solid particles permits the solid particles to move freely, thus giving the bed of solid particles liquid-like characteristics.
The state of fluidization in a fluidized bed combustion system depends mainly upon the bed-particle diameter and the velocity of the fluidizing gas. At relatively low fluidizing gas velocities and with coarse bed-particle size, the fluidized bed is relatively dense with a relatively uniform solids concentration, and has a well defined surface. This is commonly referred to as a bubbling fluidized bed (BFB), because the fluidizing gas in excess of that required to fluidize the bed passes through the bed in the form of bubbles. The bubbling fluidized bed is further characterized by a modest bed solids mixing rate, and a relatively low entrainment of solids in subsequent flue gases. At higher fluidizing gas velocities and with finer bed-particle size, the fluidized bed surface becomes more diffuse as the entrainment of solids in the flue gases increases, such that there is no longer a well defined bed surface. At still higher fluidizing gas velocities, substantially complete entrainment of bed solids occurs and recycling of the entrained material to the bed is required in order to maintain bed-particle inventory. Also, with increasing fluidizing gas velocities, the bulk density of the bed decreases with height in the furnace volume. A fluidized bed with these characteristics is referred to as a circulating fluidized bed (CFB) because of the high rate of bed material recirculation.
Inasmuch as the subject of the present application is directed to bubbling fluidized bed combustion systems, the discussion hereinafter will be presented in that context. Fluidized bed combustion systems, including, but not limited to, bubbling fluidized bed (BFB) combustion systems, are normally intended to be operative to produce steam. Moreover, such production of steam results from the combustion of fuel and air within the furnace volume of a fluidized bed combustion system. Furthermore, the steam that is so produced is designed to be operative to function in accordance with a preselected thermodynamic steam cycle.
The design of fluidized bed combustion systems is generally such that, for the purposes of the combustion that takes place therewithin, fuel is burned in a bed of hot, incombustible particles, the latter particles being suspended by the upward flow of a fluidizing gas. Moreover, this fluidizing gas is comprised of both air, which is being supplied to the fluidized bed combustion system to support the combustion of fuel therewithin, and, if need be, the gaseous byproducts which result from the combustion of the fuel and air. The combustion is accomplished in a furnace volume. A bubbling fluidized bed combustion system includes a furnace volume, the walls of which are comprised of vertical waterwall tubes. In the lower segment of the furnace volume, fuel, and possibly sorbent are mixed with and burned in air and the aforesaid fluidizing gas, producing thereby hot combustion gases which rise within the furnace volume. As the hot combustion gases, more commonly referred to now as flue gases, rise within the furnace volume, heat is transferred therefrom to the waterwall tubes, integral to the thermodynamic steam cycle, causing steam to be evaporatively produced from water rising within the waterwall tubes. A mixture of steam and water is conveyed from the upper segment of the waterwall tubes to a steam drum for separation therein. From the steam drum, the water portion of the water/steam mixture is returned to the waterwall tubes in the lower segment of the furnace volume completing a water-evaporative steam loop. In addition, it may be such that similar, additional water-evaporative steam loops are utilized in the thermodynamic steam cycle. In either case the steam portion of the water/steam mixtures is conveyed from the steam drum to other components of the thermodynamic steam cycle to which reference will be made hereinafter.
Continuing with the description of the flow of the aforesaid flue gases through the bubbling fluidized bed steam combustion system, it is noted that depending upon the needs of the thermodynamic steam cycle, the upper segment of the furnace volume may be divided into multiple chambers, each separately in fluid communication with the lower segment of the furnace volume and operative to allow the flow of flue gases therethrough. Said multiple chambers contain additional heat exchange means for the purpose of superheating and reheating, i.e., further superheating, steam as part of the thermodynamic steam cycle. Further reference will be made to this superheating and reheating momentarily.
From the top of the furnace volume, the flue gases still contain useful energy and are directed to a backpass volume wherein still further heat exchange means are located. These heat exchange means typically comprise economizer heat exchange means and have flowing therethrough water, condensed from steam expanded in a turbine. This water is heated due to an exchange of heat that takes place between the still relatively hot flue gases flowing through the backpass volume and the relatively cool economizer heat exchange means disposed therein. The now heated water is then conveyed from the economizer heat exchange means to the steam drum for continued use in the thermodynamic steam cycle. The flue gases during the passage thereof through the backpass volume are cooled as a consequence of the exchange of heat that takes place between the still relatively hot flue gases and the relatively cool economizer heat exchange means. Upon exiting the backpass volume the now cooler flue gases are commonly made to flow to an air preheater wherein air is heated prior to use in the aforesaid combustion process in the lower segment of the furnace volume. Thereafter, the flue gases are commonly made to flow, in known fashion, to and through a flue gas cleaning apparatus after which the flue gases are emitted to the atmosphere via a stack. The latter completes the description of the flue gas flow path in the bubbling fluidized bed combustion system.
Returning to the description of the fluid circuitry of the thermodynamic steam cycle, the steam portion of the water/steam mixtures is typically conveyed from the steam drum to a plurality of steam cooled backpass wall tubes which define the backpass volume. The aforesaid steam, during passage thereof through the steam cooled backpass wall tubes, is superheated as a consequence of an exchange of heat that takes place between the steam and the relatively hot flue gases flowing through the backpass volume. Upon exiting the steam cooled backpass wall tubes the now superheated steam is made to flow to a first chamber of the aforementioned multi-chambered upper segment of the furnace volume. Said steam is then made to flow through a plurality of superheat heat exchange means disposed therein and operative to further superheat the steam. The superheated steam is thence made to flow to a high pressure turbine (HPT) for expansion therein. After expansion within the high pressure turbine the still superheated steam is made to flow to a second chamber of the aforementioned multi-chambered upper segment of the furnace volume. The still superheated steam is then made to flow through a plurality of reheat heat exchange means disposed therein and operative to again superheat the steam. The again superheated steam, now commonly referred to as reheated steam, is thence made to flow to a low pressure turbine (LPT) for expansion therein. After expansion in the low pressure turbine the reheated steam is still in a superheated state and is thence made to flow to a condenser where the steam condenses to water. The water is thence made to flow, via conventional fluid flow means, from the condenser to the economizer heat exchange means located in the backpass volume. The water is heated in the economizer heat exchange means as a consequence of an exchange of heat that takes place between the aforesaid water and the still relatively hot flue gases passing through the backpass volume. The heated water is thence made to flow from the economizer heat exchange means to the steam drum for further use in the thermodynamic steam cycle. It is noted that it may also be the case that the aforesaid additional heat exchange means, located in the lower segment of the furnace volume, are made part of the water-superheated steam loop. The foregoing then completes the description of the fluid circuitry of the thermodynamic steam cycle in the bubbling fluidized bed combustion system.
It should therefore be obvious from the foregoing that the production of steam from a bubbling fluidized bed combustion system involves both a combustion process and a thermodynamic steam cycle acting in cooperative association therebetween.
The prior art is replete with examples of the use of fluidized bed combustion systems in the generation of steam. Such examples include, but are not limited to, circulating fluidized bed (CFB) systems, pressurized fluidized bed (PFB) systems and internal circulating fluidized bed (ICFB) systems as well as bubbling fluidized bed (BFB) systems. Representative of such fluidized bed combustion systems is U.S. Pat. No. 5,138,982, entitled "Internal Circulating Fluidized Bed Type Boiler And Method Of Controlling The Same," which issued on Aug. 18, 1992 and relates to an apparatus for incinerating coal, anthracite, coal dressing sludge, petro coke, bark, bagasse, industrial waste, municipal waste and other combustibles using a so-called circulating type fluidized bed as well as for recovering thermal energy from the fluidized bed, and a method of controlling the amount of diffusion gas to be blown into a thermal energy recovery chamber and the amount of fuel to be supplied in order to regulate the amount of thermal energy recovered and to maintain a constant temperature in the primary incinerating chamber of the fluidized bed. Also in the prior art, U.S. Pat. No. 5,146,878, entitled "Boiler And A Supported Heat Transfer Bank," issued on Sep. 15, 1992 and discloses a boiler, having a reaction chamber with heat transfer panels or tube banks, formed by several horizontal heat transfer tubes attached one on top of the other, and in which the ends of the heat transfer panels or the tube banks are supported by two opposing walls defining the reaction chamber. Still further in the prior art, U.S. Pat. No. 5,203,159, entitled "Pressurized Fluidized Bed Combustion Combined Cycle Power Plant and Method of Operating The Same," issued on Apr. 20, 1993, and is directed to a pressurized fluidized bed combustion combined cycle power plant and method of operating the same; more particularly to a combined cycle power plant comprising a pressurized fluidized bed combustion boiler, combustion boiler for burning coal and producing steam, a gas turbine, a compressor and a generator, with the steam being usable to drive a steam turbine and with means being provided for maintaining a stable fluidization in the pressurized fluidized bed combustion boiler even at partial load. Yet further in the prior art, U.S. Pat. No. 5,255,507, entitled "Combined Cycle Power Pant Incorporating Atmospheric Circulating Fluidized Bed Boiler And Gasifier," issued on Oct. 26, 1993 and relates to circulating fluidized bed boilers and pertains particularly to a combined cycle power plant incorporating an atmospheric circulating fluidized bed boiler and gasifier. Further representative of the prior art with respect to the use of fluidized bed combustion systems in the generation of steam is U.S. Pat. No. 5,273,000, entitled "Reheat Steam Temperature Control In A Circulating Fluidized Bed Steam Generator." U.S. Pat. No. 5,273,000, issued on Dec. 28, 1993, is assigned to the same assignee as the present invention and discloses a method for the fluidized bed combustion of a fuel in a circulating fluidized bed system and particularly to a method for controlling the extraction of heat from the recycle solids to control the temperature of a fluid such as reheated steam. Still further in the prior art, U.S. Pat. No. 5,471,955, entitled "Fluidized Bed Combustion System Having A Heat Exchanger In The Upper Furnace," issued on Dec. 5, 1995 and relates to a fluidized bed combustion system and method, and, more particularly, to such a system and method in which a heat exchanger is provided in the upper portion of the furnace. Yet further in the prior art, U.S. Pat. No. 5,533,471, entitled "Fluidized Bed Reactor And Method Of Operation Thereof," issued on Jul. 9, 1996 and relates to controlled operation of a circulating fluidized bed reactor that has a number of advantages compared to prior art construction and processes.
More specifically, the prior art also reveals reference to bubbling fluidized bed combustion systems for use in the generation of steam. In particular, U.S. Pat. No. 4,103,646, entitled "Apparatus And Method For Combusting Carbonaceous Fuels Employing In Tandem a Fast Bed Boiler And A Slow Bed Boiler," issued on Aug. 1, 1978 and discloses an invention in which a fluid bed boiler and combustion method are provided having two zones. Still further in the prior art, U.S. Pat. No. 4,325,327, entitled "Hybrid Fluidized Bed Combustor," issued on Apr. 20, 1982 and is assigned to the same assignee as the present invention and embodies a system which includes a furnace whose source of heat is an atmospheric bubbling fluidized bed burning crushed solid fuel. Yet further in the prior art, U.S. Pat. No. 5,526,775, entitled "Circulating Fluidized Bed Reactor And Method Of Operating The Same," issued on Jun. 18, 1996 and relates to a circulating fluidized bed reactor having substantially vertical walls with cooling elements therein, the vertical walls defining the interior of the reactor chamber; means for introducing fluidizing gas at the bottom of the fluidizing bed reactor; means for introducing particulate matter into said reactor; separator for separating particulate material from the gases, the separator being in connection with said reactor at the upper section thereof; return duct, being connected to the separator; bubbling fluidized bed adjacent to the reactor and being provided with heat exchanger means for cooling particulate material, side walls, and rear and front wall shaving cooling elements in fluid communication with the cooling elements of the reactor, said bubbling fluidized bed being connected with said return duct.
Fluidized bed combustion systems are known to be flexible in their ability to burn a wide variety of fuel types. Included in these fuels is pulverized coal, anthracite, sludge, petro coke, bagasse, bark, and industrial and municipal wastes. Representative of the utilization of such fuel types in fluidized bed combustion systems is found in U.S. Pat. No. 5,138,958, entitled "Process For Incinerating Domestic Refuse In A Fluidized Bed Furnace," which issued on Aug. 18, 1992. In accordance with the teachings of U.S. Pat. No. 5,138,958, a process for incinerating domestic refuse is implemented in a boiler comprising a fluidized bed furnace over which is a post-combustion chamber. Also in the prior art, reference is again made to U.S. Pat. No. 5,138,982. Yet further in the prior art, U.S. Pat. No. 5,156,099 entitled, "Composite Recycling Type Fluidized Bed Boiler," issued on Oct. 20, 1992 and relates to an internal recycling type fluidized bed boiler in which combustion materials such as various coals, low grade coal, dressing sludge, oil cokes and the like are burnt by a so-called whirling flow fluidized bed, the interior of a freeboard and a heat transfer portion provided downstream of the freeboard portion. Still further in the prior art U.S. Pat. No. 5,189,964, entitled "Process For Burning High Ash Particulate Fuel," issued on Mar. 2, 1993 and relates to fuel combustion in stationary power generating plants; and more particularly to particulate carbonaceous fuel and the use thereof in fluidized-bed combustion processes. Continuing in the prior art with respect to the burning of various types of fuel in fluidized bed combustion systems, U.S. Pat. No. 5,365,889, entitled "Fluidized Bed Reactor And System And Method Utilizing Same," issued on Nov. 22, 1994 and relates to an improved fluidized bed reactor and method, and more particularly, to a fluidized bed reactor and method for incinerating combustible materials such as municipal and industrial wastes. Yet further in the prior art, U.S. Pat. No. 5,395,596, entitled "Fluidized Bed Reactor And Method Utilizing Refuse Derived Fuel," issued on Mar. 7, 1995 and relates to a fluidized bed reactor and method of operating a fluidized bed reactor and, more particularly, to such a reactor and method in which the reactor is fueled in whole or in part by refuse derived fuel, or RDF. The prior art with respect to the utilization of a wide variety of fuels in fluidized bed combustion systems also reveals U.S. Pat. No. 4,449,482, entitled "Fluidized Bed Boilers." U.S. Pat. No. 4,449,482 issued on May 22, 1984 and relates to fluidized bed boilers. In accordance with the teachings of U.S. Pat. No. 4,449,482, in reactors generating hot gases, air is passed through a bed of particulate material which includes a mixture of inert material and a fuel material such as coal, wood waste or other combustible materials. Yet still further the prior art shows U.S. Pat. No. 4,823,712, entitled "Multifuel Bubbling Bed Fluidized Bed Combustion System," issued on Apr. 25, 1989 and relates to providing multifuel capability for a fluidized bed combustor and related methods and apparatus for combusting low-BTU fuels and generating high-temperature gases while reducing environmental pollutants. The prior art relating to the burning of various fuel types in fluidized bed combustion systems also reveals U.S. Pat. No. 5,313,913, entitled "Pressurized Internal Circulating Fluidized-Bed Boiler" which issued on May 24, 1994 and relates to a pressurized fluidized-bed combined cycle electric generating system in which a fuel such as coal, petro coke, or the like is combusted in a pressurized fluidized bed and an exhaust gas produced by the combusted fuel is introduced into a gas turbine. Continuing in the prior art, U.S. Pat. No. 5,513,599, entitled "Pressurized Internal Circulating Fluidized Bed Boiler," issued on May 7, 1996 and relates to a pressurized fluidized-bed boiler, and more particularly to a pressurized internal circulating fluidized-bed boiler for use in a pressurized fluidized-bed combined-cycle electric generating system in which a fuel such as coal, petro coke, or the like is combusted in a pressurized fluidized bed and an exhaust gas produced by the combusted fuel is introduced into a gas turbine.
It has been long known and practiced in the prior art to utilize oil bearing shale in a retorting process, i.e., in the extraction of various oils, gases and vapors therefrom. By driving off and then condensing volatiles found in the shale, light petroleum products such as gasoline may be derived from the shale. However, such a retorting process is distinct from the use of shale as a fuel in a fluidized bed combustion system wherein the entirety of the shale is burned and byproduct thereof are not sought. Representative of the prior art with respect to oil shale retorting is seen in U.S. Pat. No. 1,451,575, entitled "Oil Shale Retort," which issued on Apr. 10, 1923. In accordance with the teachings of U.S. Pat. No. 1,451,575 the invention therein relates to an apparatus for and process of distilling or separating the oil contained in oil shale, asphalt or other oil bearing mineral, from the solid or residual matter thereof and is particularly designed for use in refining plants in which oil shale is treated for the purpose of extracting its oil contents. Still further in the prior art, U.S. Pat. No. 1,465,277, entitled "Apparatus For Extracting Oil From Shales," issued on Aug. 21, 1923 and relates to a process, mechanism and retort adapted to receive crushed or powdered shale in suitable quantities, to heat and drive off the oil elements as a gas undeprived of its qualities, the gas being later condensed and stored as oil in containers such as may be provided. Continuing in the prior art, U.S. Pat. No. 2,774,726, entitled "Apparatus For The Recovery Of Oil And Gaseous Products From Shale," issued on Dec. 18, 1956 and relates to a method and apparatus for the recovery of oil and gaseous products from shale. Yet further in the prior art, U.S. Pat. No. 4,409,092, entitled "Combination Process For Upgrading Oil Products Of Coal, Shale Oil And Crude Oil To Produce Jet Fuels, Diesel Fuels And Gasoline," issued on Oct. 11, 1983 and relates to a method and combination of processing steps for producing jet fuels from hydrocarbon materials derived from oil shale, coal and crude oil. Further in the prior art, U.S. Pat. No. 4,438,816, entitled "Process For Recovery Of Hydrocarbons From Oil Shale," issued on Mar. 27, 1984 and relates to the processing of oil shale in a manner in which allows the recovery of a high percentage of the hydrocarbonaceous oil which is contained therein.
The use of oil shale in conjunction with fluidized bed systems is also found in the prior art. U.S. Pat. No. 4,373,454, entitled "Oil Shale Retorting And Combustion System," issued on Feb. 15, 1983 and relates to a fluidized bed system in which oil shale containing calcium carbonate is subject to sequentially occurring (two-stage) retorting and combustion steps whereby volatile hydrocarbons representing maximum heat value may be extracted from the oil shale prior to the occurrence of any calcination reaction so as to extensively increase the efficiency of the energy extraction process relating to oil shale. Further in the prior art, U.S. Pat. No. 4,481,080, entitled "Staged Fluidized Combustion," issued on Nov. 6, 1984 and relates to oil shale retorting and more particularly to staged fluidized bed oil shale retorting.
To summarize, it should be evident from the foregoing broad array of prior art that fluidized beds are utilized in numerous capacities, including the generation of steam. In particular, circulating fluidized beds, pressurized fluidized beds, internal circulating fluidized beds and bubbling fluidized beds are but a few of the fluidized bed combustion systems utilized in the generation of steam. Furthermore, the prior art shows that fluidized bed combustion systems, while utilized in the generation of steam, are capable of burning a wide variety of fuels including pulverized coal, anthracite, sludge, petro coke, bagasse, bark, and industrial and municipal wastes. Still further, the prior art shows that oil shale is utilized for purposes other than in the generation of steam, such as in the extraction therefrom of various oils, gases and vapors in contexts other than as a fuel in fluidized bed combustion systems, e.g., in retorting oil shale.
However, in canvassing the prior art it should be readily apparent that, although fluidized bed combustion systems, when utilized in the generation of steam and constructed in accordance with the teachings of the U.S. patents to which reference has been made, have been demonstrated to be operative for the purpose for which they were designed, there has nevertheless been evidenced in the prior art that there exists a need for such fluidized bed combustion systems to be improved. More specifically, a need has been evidenced in the prior art relating to the generation of steam by fluidized bed combustion systems that there exists a need for a new and improved fluidized bed combustion system in the nature of a bubbling fluidized bed combustion system capable of effecting therein the combustion of a highly reactive fuel having a high ash content, a low higher heating value (HHV), a high limestone (CaCO.sub.3) content and having a tendency to foul convective heat transfer surfaces.
Moreover, such a new and improved bubbling fluidized bed combustion system, capable of utilizing shale oil as a fuel therein and generating steam thereby, would be particularly characterized in a number of respects. To that end, one such characteristic which such a new and improved bubbling fluidized bed combustion system would desirably possess is that of a bubbling bed of hot solids disposed within the lower segment of the furnace volume. It is also desirable that such a bubbling bed of hot solids include therein a plurality of isolatable segments. Furthermore, it is desirable that the bubbling bed of hot solids be capable of being slumped, i.e., capable of having one or more of said segments thereof isolated and made inoperative for the purpose of operating the bubbling fluidized bed combustion system so as to effectively respond to changing demands placed thereupon. Still further, it is desirable that such a bubbling bed of hot solids be capable of containing, in such a plurality of isolatable segments thereof, a plurality of heat exchange means for use either in a water-evaporative steam loop, or in a water-superheated steam loop in the thermodynamic steam cycle. By so incorporating such heat exchange means in the isolatable segments of the bubbling bed of hot solids, flue gas temperatures in the furnace volume may be effectively controlled. By so controlling the flue gas temperatures the bubbling fluidized bed combustion system may be operated at a lower flue gas temperature, thus reducing the aforesaid fouling of convective heat exchange means and reducing the tendency of the bed solids to agglomerate.
Still further, a characteristic which such a new and improved bubbling fluidized bed combustion system would desirably possess is that of a deentrainment zone disposed in the lower segment of the furnace volume. Such a deentrainment zone would be characterized by an increasing cross sectional area with increasing height within the furnace volume such that larger particulate matter, i.e., larger unburned oil shale particles entrained within the upwardly mobile flue gases, fall out of the flue gas stream and are recirculated to the bubbling bed of hot solids for continued combustion. Such a deentrainment zone also increases the residence time of a particle of fuel, thereby providing for the greater likelihood of combustion of a particle of fuel. Furthermore, such a deentrainment zone reduces the amount of fly ash carried away in the flue gas stream, thus reducing the amount of fouling of convective heat transfer surfaces as well as increasing sulfur capture and carbon burnout.
Furthermore, a characteristic which such a new and improved bubbling fluidized bed combustion system would desirably possess is that of the presence of freeboard heat exchange means disposed in the lower segment of the furnace volume above the deentrainment zone. Such freeboard heat exchange means should be capable of heat transfer duty either in a water-evaporative steam loop, or in a water-superheated steam loop in the thermodynamic steam cycle. Furthermore, such freeboard heat exchange means are disposed parallel to the flow of the upwardly mobile flue gases and act to remove heat therein released thereto due to the combustion of oil shale. The freeboard heat exchange means also provide effective control of the temperature of the upwardly mobile flue gases in the furnace volume so as to allow the fluidized bed combustion system to be operated at a lower flue gas temperature, thus reducing the potential for the aforesaid fouling of convective heat exchange means.
Another characteristic which such a new and improved bubbling fluidized bed combustion system would desirably possess is that of a fluidized bed ash cooler incorporated as an integral part thereof wherein the fluidized bed ash cooler acts as a heat exchanger. It is desirable that such a fluidized bed ash cooler be capable of accepting as input thereto relatively hot ash particles, originating from the bubbling bed of hot solids and resulting from the incomplete combustion of oil shale within the furnace volume. Still further, such a fluidized bed ash cooler should be capable of accepting as fluid input thereto relatively cool air, for exchanging heat therein with the aforesaid relatively hot ash particles. Thereupon, the now hotter air is delivered therefrom to either the aforesaid bubbling bed of hot solids or to an overbed air inlet means disposed above the bubbling bed of hot solids in the lower segment of the furnace volume. Such an exchange of heat between the air and the relatively hot ash recovers still useful energy from the relatively hot ash for further use in the flue gas flow path or the thermodynamic steam cycle, thus improving the efficiency of the bubbling fluidized bed combustion system. Furthermore, it is desirable that such a fluidized bed ash cooler be capable of classifying finer ash particles from coarser ash particles and returning the finer ash particles directly to the bubbling bed of hot solids while yet delivering the coarser ash particles to an ash discharge means. Yet further, it is desirable that such a fluidized bed ash cooler be capable of performing heat transfer duty either in a water-evaporative steam loop, or in a water-superheated steam loop in the aforesaid thermodynamic steam cycle. It is also desirable that such a fluidized bed ash cooler include therein a plurality of isolatable segments thereof. Still further, it is desirable that such a fluidized bed ash cooler be capable of being slumped, i.e., capable of having one or more of said segments thereof isolated and made inoperative for the purpose of operating the bubbling fluidized bed combustion system so as to effectively respond to changing demands placed thereupon, while yet maintaining the aforesaid classification capability.
Yet another characteristic which such a new and improved bubbling fluidized bed combustion system would desirably possess is that of a multi-chambered upper segment of the furnace volume; each chamber therein being in fluid communication with the lower segment of the furnace volume. In particular it is desirable that such multi-chambered upper segment of the furnace volume comprise for example two chambers whereby a first chamber thereof contains a plurality of superheat heat exchange means in fluid communication therebetween and integral to the aforesaid thermodynamic steam cycle and a second chamber thereof contains a plurality of reheat heat exchange means in fluid communication therebetween and integral to the thermodynamic steam cycle. Still further it is desirable that such a multi-chambered upper segment of the furnace volume have operatively connected thereto, means for controlling the distribution of the flow of the aforesaid upwardly mobile flue gases between and through the first and second chambers thereof and controlling thereby the reheated steam outlet (RHO) temperature of the thermodynamic steam cycle.
Another desirable characteristic that such a new and improved bubbling fluidized bed combustion system would possess is that of an underbed air inlet means connected to the lower segment of the furnace volume and operative to properly fluidize the bubbling bed of hot solids by delivering upwardly mobile fluidizing air to the fluidized bed at a prescribed volumetric flow rate. By so controlling the volumetric flow rate of the fluidizing air, proper control may be had of the local stoichiometry of the combustion process, thus maintaining proper flue gas temperatures. Furthermore, such control of the volumetric flow rate of the fluidizing air helps prevent agglomeration of bed solids by mechanical agitation thereof.
Still further it is desirable that such a bubbling fluidized bed combustion system have an overbed air inlet means operative to deliver overbed air to the furnace volume to support the combustion of oil shale therein. By the judicious manipulation and control of such combustion supporting air and the upwardly flowing fluidizing air, proper control may then be had of the stoichiometric ratio of the combustion process in the furnace volume, thus controlling NO.sub.x emissions therefrom.
It is also desirable that such a new and improved bubbling fluidized bed combustion system possess an overbed fuel inlet means disposed in the lower segment of the furnace volume above the bubbling bed of hot solids and proximate in elevation with the overbed air inlet means. Said overbed fuel inlet means is operative to spread oil shale evenly over the bubbling bed of hot solids.
Still another desirable characteristic that such a new and improved bubbling fluidized bed combustion system would possess is that of an ash drainage means disposed in the lower segment of the furnace volume proximate in elevation with the bubbling bed of hot solids while yet further disposed opposite from the overbed fuel inlet means in the furnace volume.
Yet further, it is desirable that such a new and improved bubbling fluidized bed combustion system be capable of the combustion of oil shale therein and the generation of steam thereby without the need for incorporating in the fluidized bed combustion system a solids/gas separator.
Still further, it is desirable that such a new and improved bubbling fluidized bed combustion system have a backpass volume in fluid communication with the furnace volume and contain therein a plurality of economizer heat exchange means in fluid communication therebetween and integral to the thermodynamic steam cycle. It is also desirable that such a backpass volume contain an air preheater means for heating air prior to injection thereof into the furnace volume via the underbed air inlet means.
It is therefore an object of the present invention to provide a new and improved fluidized bed combustion system. To that end it is an object of the present invention to provide such a new and improved fluidized bed combustion system that is in the nature of a bubbling fluidized bed combustion system which is particularly well suited to effect therein the combustion of a highly reactive fuel having a high ash content, a low higher heating value (HHV), a high limestone (CaCO.sub.3) content and having a tendency to foul convective heat transfer surfaces.
It is further an object of the present invention to provide such a new and improved fluidized bed combustion system, particularly well suited to effect therein the combustion of oil shale, so as to have a bubbling bed of hot solids disposed within the lower segment of the furnace volume. It is also an object of the present invention to provide such a bubbling bed of hot solids including therein a plurality of isolatable segments whereby such a bubbling bed of hot solids which is capable of being slumped, i.e., having one or more segments thereof isolated and made inoperative for the purpose of operating the bubbling fluidized bed combustion system so as to effectively respond to changing demands placed thereupon. Still further, it is an object of the present invention that such a bubbling bed of hot solids be capable of containing, in such a plurality of isolatable segments thereof, a plurality of heat exchange means for use either in a water-evaporative steam loop, or in a water-superheated steam loop in the thermodynamic steam cycle.
It is also an object of the present invention to provide such a new and improved fluidized bed combustion system, particularly well suited to effect therein the combustion of oil shale, so as to have a deentrainment zone disposed in the lower segment of the furnace volume and characterized by an increasing cross sectional area with increasing height within the furnace volume such that larger particulate matter, i.e., larger unburned fuel particles entrained within the upwardly mobile flue gases, fall out of the flue gases and are recirculated to the bubbling bed of hot solids for continued combustion.
Yet a further object of the present invention is to provide such a new and improved bubbling fluidized bed combustion system, particularly well suited to effect the combustion therein of oil shale, having freeboard heat exchange means disposed in the lower segment of the furnace volume above the deentrainment zone. It is an object of the present invention that such freeboard heat exchange means are capable of performing heat transfer duty either in a water-evaporative steam loop, or in a water-superheated steam loop in the thermodynamic steam cycle. Furthermore, it is an object of the present invention that such freeboard heat exchange means be disposed parallel to the flow of the upwardly mobile flue gases and operative to remove heat therefrom released thereinto due to the combustion of shale oil, providing effective control thereby of the temperature of the upwardly mobile flue gases in the furnace volume. Such control of the temperature of the upwardly mobile flue gases allows the fluidized bed combustion system to be operated at a lower flue gas temperature, thereby reducing the potential for the aforesaid fouling of convective heat exchange means that arises with the use of oil shale as a fuel.
Yet further, it is an object of the present invention to provide such a new and improved fluidized bed combustion system, particularly well suited to effect therein the combustion of oil shale, so as to have a fluidized bed ash cooler incorporated as an integral part thereof wherein the fluidized bed ash cooler acts as a heat exchanger. It is also an object of the present invention to provide such a fluidized bed ash cooler that is capable of accepting as input thereto relatively hot ash particles, originating from the bubbling bed of hot solids and resulting from the incomplete combustion of shale oil within the furnace volume. It is also an object of the present invention to provide such a fluidized bed ash cooler which is capable of accepting as fluid input thereto relatively cool air, for exchanging heat therein with the aforesaid relatively hot ash particles, prior to delivery therefrom to either the aforesaid bubbling bed of hot solids or to an overbed air inlet means disposed above the bubbling bed of hot solids in the lower segment of the furnace volume. It is also an object of the present invention to provide such a fluidized bed ash cooler so as to be capable of classifying finer ash particles from coarser ash particles and returning the finer ash particles either to the bubbling bed of hot solids while yet delivering the coarser ash particles to an ash discharge means. Still further it is an object of the present invention to provide such a fluidized bed ash cooler which is capable of performing heat transfer duty either in a water-evaporative steam loop, or in a water-superheated steam loop in the thermodynamic steam cycle. It is also an object of the present invention that such a fluidized bed ash cooler include therein a plurality of isolatable segments thereof. Yet further it is an object of the present invention to provide such a fluidized bed ash cooler that is capable of being slumped, i.e., having one or more segments thereof isolated and made inoperative for the purpose of operating the bubbling fluidized bed combustion system so as to effectively respond to changing demands placed thereupon while yet maintaining the aforesaid classification capability.
Yet further, it is an object of the present invention to provide such a new and improved fluidized bed combustion system, particularly well suited to effect therein the combustion of oil shale, so as to have a multi-chambered upper segment of the furnace volume;
each chamber therein being in fluid communication with the lower segment of the furnace volume. It is also an object of the present invention to provide such a multi-chambered upper segment of the furnace volume so as to comprise for example two chambers whereby a first chamber thereof contains a plurality of superheat heat exchange means in fluid communication therebetween and integral to the thermodynamic steam cycle and a second chamber thereof contains a plurality of reheat heat exchange means in fluid communication therebetween and integral to the thermodynamic steam cycle. Still further, it is an object of the present invention to provide such a multi-chambered upper segment of the furnace volume so as to have operatively connected thereto, means for controlling the distribution of the flow of the aforesaid upwardly mobile flue gases through the first and second chambers thereof and controlling thereby the reheated steam outlet (RHO) temperature of the thermodynamic steam cycle.
It is another object of the present invention to provide such a new and improved bubbling fluidized bed combustion system, particularly well suited to effect therein the combustion of oil shale, so as to have an underbed air inlet means connected to the lower segment of the furnace volume and operative to properly fluidize the bubbling bed of hot solids by delivering upwardly mobile fluidizing air to the fluidized bed at a prescribed volumetric flow rate.
It is another object of the present invention to provide such a new and improved bubbling fluidized bed combustion system, particularly well suited to effect therein the combustion of oil shale, so as to have an overbed air inlet means disposed in the lower segment of the furnace volume above the bubbling bed of hot solids and operative to control the stoichiometric ratio of the combustion process in the furnace volume; thus controlling NO.sub.x emissions therefrom.
It is also an object of the present invention that such a new and improved bubbling fluidized bed combustion system possess an overbed fuel inlet means disposed in the lower segment of the furnace volume above the bubbling bed of hot solids and operative to spread oil shale evenly over the bubbling bed of hot solids.
Still further it is an object of the present invention to provide such a new and improved bubbling fluidized bed combustion system, particularly well suited to effect therein the combustion of oil shale, having an ash drainage means disposed in the lower segment of the furnace volume proximate in elevation with the bubbling bed of hot solids yet disposed opposite from the overbed fuel inlet means.
It is also an object of the present invention to provide such a new and improved bubbling fluidized bed combustion system which is particularly well suited for effecting therein the combustion of oil shale without incorporating in the fluidized bed combustion system a solids/gas separator.
Furthermore, it is an object of the present invention to provide such a new and improved bubbling fluidized bed combustion system, particularly well suited to effect therein the combustion of oil shale, having a backpass volume in fluid communication with the furnace volume and containing a plurality of economizer heat exchange means in fluid communication therebetween and integral to the thermodynamic steam cycle. It is also an object of the present invention that such a backpass volume contain an air preheater means for heating air prior to injection thereof into the furnace volume via the underbed air inlet means.